1. Field of the Invention
The present invention generally relates to DC brushless motors and devices for driving the same. More particularly, the present invention is concerned with a DC brushless motor suitable for driving a high-speed polygon mirror used to scan a light beam in an image forming device such as a laser beam printer, a digital copying machine or a digital facsimile machine.
2. Description of the Related Art
A DC brushless motor driving device for controlling the rotation velocity of a DC brushless motor is known in which currents flowing in driving coils of the DC motor are turned ON and OFF with a period less than the period of a rotor position detecting signal. Such a DC brushless motor driving device is disclosed in, for example, Japanese Laid-Open Patent Application No. 58-99289. The proposed driving device is principally capable of efficiently driving the DC motor. A conventional DC brushless motor driving device such as disclosed in the above application will now be described in more detail by referring to FIG. 1.
FIG. 1 is a block diagram of the overall structure of such a conventional DC brushless motor driving device. The driving device shown in FIG. 1 is made up of a velocity control circuit 101, a pulse width modulation circuit 102, a current flow switching circuit 103, a switching circuit 104, and a current smoothing circuit 105. The velocity control circuit 101 compares the current velocity obtained from one of three position detecting signals with a target velocity, and produces the difference therebetween as a velocity control voltage. The pulse width modulation circuit 102 modulates the pulse width of a velocity control pulse signal in accordance with the velocity control voltage supplied from the velocity control circuit 101.
The current smoothing circuit 105 smooths the velocity control pulse signal from the pulse width modulation circuit 102 and produces a resultant DC driving current. The switching circuit 104 distributes the driving current to the driving coils of a DC brushless motor 106 having three phases in accordance with current-flow switching signals produced by the current-flow switching circuit 103. The circuit 103 derives the current-flow switching signals from the three position detecting signals from the motor 106.
The current smoothing circuit 105 is utilized due to consideration of the following operating characteristic of the motor.
The high-frequency switching operation on the current sequentially flowing in the driving coils of the motor 106 results in a counter electromotive force when the switching is turned OFF. The counter electromotive force can be expressed as follows: EQU V=-L(di/dt) (1)
where V is the counter electromotive force, L is the inductance of the driving coil, and i is the current flowing in the driving coil.
The counter electromotive force V, produced when the supply of the current flowing in the driving coil of the motor is stopped, is increased as the switching speed (frequency) is increased. In practice, the counter electromotive force V may pass through a diode used to protect switching elements formed of bipolar transistors in the counter direction and may flow into a power supply. The above counter current resulting from the counter electromotive force V causes the driving device to malfunction.
The current smoothing circuit 105 is used to smooth the velocity control pulse signal which is a high-frequency signal and produce the resultant DC current. Hence, it is possible to control the counter current to flow into the power supply and ensure normal driving operation.
FIG. 2 is a circuit diagram of the current smoothing circuit 105 shown in FIG. 1. The current smoothing circuit 105 shown in FIG. 2 is made up of a choke coil CH, a diode D and a capacitor C. The combination of these parts of the circuit 105 function to smooth the velocity control pulse signal and output the smoothed driving current (DC current).
However, use of the choke coil CH and the capacitor C increases the production cost. Further, the choke coil CH and the capacitor C are large-size elements, which prevent down-sizing of the driving device.
Referring to FIG. 3, a motor scanner 120 includes the DC motor 106 and a polygon mirror 110. The motor scanner 120 is controlled by a DC motor driving device 107 configured as shown in FIG. 1 and connected to the motor scanner 120 via a harness 109 or the like. Since the driving device 107 includes the current smoothing circuit 105, it is necessary to separately form the motor scanner 120 and the driving circuit 107. The driving circuit 107 shown in FIG. 3 is implemented by two IC devices IC1 and IC2 mounted on a circuit board 108B as well as the choke coil CH, the capacitor C and the diode D of the current smoothing circuit 105 is also mounted thereon.